1. Field of the Invention
The present invention relates to an electronic still camera, and in particular to a micro mode executing apparatus of a digital still camera, in which an image sensor and a focus controlling lens are connected to a focus driving motor and a spindle of the motor, thereby controlling an optical length within a range of not changing a barrel structure.
2. Description of the Prior Art
Recently, while electronic still cameras recording a still image on a magnetic disc in a form of an electrical image signal have been developed, the electronic still camera has to control the focusing by moving a focus ring of the photographing lens, such as a typical still camera using a silver-salt film.
In that case, the automatic focusing control may be performed by an automatic focus mechanism. The automatic focus mechanism detects the distance from the camera to a subject and moved the focus ring of the photographing lens based on the distance information.
Meanwhile, in a video camera, a photographing signal itself detects a focus information for an automatic focus control to perform the automatic focus control without detecting the distance. Specifically, when the focus control of the lens is proper so that the profile of the subject can be photographed distinctly, there is no high band frequency component in the photograph signal. When the focus control of the lens is not proper, since the profile of the subject is blurred, there is a high band frequency component in the signal.
Accordingly, the automatic focus control is performed by detecting the amount of a high band frequency components contained in the photographing signal to determine whether the state of the focus control is proper or not.
A zoom lens is widely used as a photographing lens mounted onto the camera. Using such a zoom lens, a focusing distance may be successively varied, so that a photography can be achieved in an optional viewing angle between a wide angle and a telescope angle.
However, in the zoom lens, if the focus distance is varied, there is a problem in that the focus position is also varied. Specifically, since it adjusts accurately the focus of the subject before varying the focus distance, although there is no variation of the distance from the camera to the subject, the focus of the subject is deviated, if the focus distance is varied.
In the typical zoom lens, in order to prevent the focus from being deviated, if a zoom ring for adjusting a focus distance rotates, a focus adjusting lens is gradually moved by components, such as a cam groove, so that the focus position may be compensated in line with the variation of the focus distance.
As shown in FIG. 1, in a high-grade camera, an application of a micro mode increases the ability to photograph an object that is close to the camera, i.e. not far off a limited distance predetermined in the lens design. Such a micro mode photographs a near object by extending a the total length of the camera optical system, while allowing degradation of the photographing performance of the lens.
With reference to FIG. 1, a position of the focus on which the object OBJ is photographed on a position of a normal lens LENS is designated by a reference numeral F1. At that time, if the lens is moved forwardly, the distance between the object OBJ and the lens is shortened to vary the position of the focus of the object to be photographed.
Accordingly, although the lens moves substantially, supposing that the position of the lens is fix as shown in FIG. I, the object OBJ moves relatively to the lens to produce a virtual image of the object designated by a reference numeral IOBJ. Therefore, the focus of the object to be photographed by the virtual image of the object is positioned as a reference numeral F2.
Specifically, in the electronic still camera, in order to move the lens from or to the image sensor, the camera has to have two lenses as shown in FIG. 2, and a transferring guide member having a barrel structure to transfer or moved.
The application of the micro mode of the conventional electronic still camera will be described with reference to FIG. 2.
The focusing mode of the conventional electronic still camera includes an object lens Lo faced to the object, and an image lens Li faced to an image sensor IS. The focusing mode is determined by transferring or moving any of the lenses.
In case of transferring the image lens Li to apply the micro mode, there is an advantage in that since the image lens and the object lens have a stable position due to the solidity of the barrel structure as shown in FIG. 2, the performance of the camera can be constantly maintained. Also, the transferred image lens is secured to have an optical stability.
However, there is a disadvantage in that since the distance FL between the image sensor and the object lens is fixed, the camera does not satisfy the anticipation of the micro mode.